IO::Handle - supply object methods for I/O handles


    use IO::Handle;
    $io = new IO::Handle;
    if ($io->fdopen(fileno(STDIN),"r")) {
        print $io->getline;
    $io = new IO::Handle;
    if ($io->fdopen(fileno(STDOUT),"w")) {
        $io->print("Some text\n");
    # setvbuf is not available by default on Perls 5.8.0 and later.
    use IO::Handle '_IOLBF';
    $io->setvbuf($buffer_var, _IOLBF, 1024);
    undef $io;       # automatically closes the file if it's open
    autoflush STDOUT 1;


IO::Handle is the base class for all other IO handle classes. It is not intended that objects of IO::Handle would be created directly, but instead IO::Handle is inherited from by several other classes in the IO hierarchy.

If you are reading this documentation, looking for a replacement for the FileHandle package, then I suggest you read the documentation for IO::File too.


new ()

Creates a new IO::Handle object.

new_from_fd ( FD, MODE )

Creates an IO::Handle like new does. It requires two parameters, which are passed to the method fdopen; if the fdopen fails, the object is destroyed. Otherwise, it is returned to the caller.


See the perlfunc manpage for complete descriptions of each of the following supported IO::Handle methods, which are just front ends for the corresponding built-in functions:

    $io->format_write( [FORMAT_NAME] )
    $io->read ( BUF, LEN, [OFFSET] )
    $io->print ( ARGS )
    $io->printf ( FMT, [ARGS] )
    $io->sysread ( BUF, LEN, [OFFSET] )
    $io->syswrite ( BUF, [LEN, [OFFSET]] )
    $io->truncate ( LEN )

See the perlvar manpage for complete descriptions of each of the following supported IO::Handle methods. All of them return the previous value of the attribute and takes an optional single argument that when given will set the value. If no argument is given the previous value is unchanged (except for $io->autoflush will actually turn ON autoflush by default).

    $io->autoflush ( [BOOL] )                         $|
    $io->format_page_number( [NUM] )                  $%
    $io->format_lines_per_page( [NUM] )               $=
    $io->format_lines_left( [NUM] )                   $-
    $io->format_name( [STR] )                         $~
    $io->format_top_name( [STR] )                     $^
    $io->input_line_number( [NUM])                    $.

The following methods are not supported on a per-filehandle basis.

    IO::Handle->format_line_break_characters( [STR] ) $:
    IO::Handle->format_formfeed( [STR])               $^L
    IO::Handle->output_field_separator( [STR] )       $,
    IO::Handle->output_record_separator( [STR] )      $\
    IO::Handle->input_record_separator( [STR] )       $/

Furthermore, for doing normal I/O you might need these:

$io->fdopen ( FD, MODE )

fdopen is like an ordinary open except that its first parameter is not a filename but rather a file handle name, an IO::Handle object, or a file descriptor number.


Returns true if the object is currently a valid file descriptor, false otherwise.


This works like <$io> described in I/O Operators in the perlop manpage except that it's more readable and can be safely called in a list context but still returns just one line. If used as the conditional +within a while or C-style for loop, however, you will need to +emulate the functionality of <$io> with defined($_ = $io->getline).


This works like <$io> when called in a list context to read all the remaining lines in a file, except that it's more readable. It will also croak() if accidentally called in a scalar context.

$io->ungetc ( ORD )

Pushes a character with the given ordinal value back onto the given handle's input stream. Only one character of pushback per handle is guaranteed.

$io->write ( BUF, LEN [, OFFSET ] )

This write is like write found in C, that is it is the opposite of read. The wrapper for the perl write function is called format_write.


Returns a true value if the given handle has experienced any errors since it was opened or since the last call to clearerr, or if the handle is invalid. It only returns false for a valid handle with no outstanding errors.


Clear the given handle's error indicator. Returns -1 if the handle is invalid, 0 otherwise.


sync synchronizes a file's in-memory state with that on the physical medium. sync does not operate at the perlio api level, but operates on the file descriptor (similar to sysread, sysseek and systell). This means that any data held at the perlio api level will not be synchronized. To synchronize data that is buffered at the perlio api level you must use the flush method. sync is not implemented on all platforms. Returns ``0 but true'' on success, undef on error, undef for an invalid handle. See fsync(3c).


flush causes perl to flush any buffered data at the perlio api level. Any unread data in the buffer will be discarded, and any unwritten data will be written to the underlying file descriptor. Returns ``0 but true'' on success, undef on error.

$io->printflush ( ARGS )

Turns on autoflush, print ARGS and then restores the autoflush status of the IO::Handle object. Returns the return value from print.

$io->blocking ( [ BOOL ] )

If called with an argument blocking will turn on non-blocking IO if BOOL is false, and turn it off if BOOL is true.

blocking will return the value of the previous setting, or the current setting if BOOL is not given.

If an error occurs blocking will return undef and $! will be set.

If the C functions setbuf() and/or setvbuf() are available, then IO::Handle::setbuf and IO::Handle::setvbuf set the buffering policy for an IO::Handle. The calling sequences for the Perl functions are the same as their C counterparts--including the constants _IOFBF, _IOLBF, and _IONBF for setvbuf()--except that the buffer parameter specifies a scalar variable to use as a buffer. You should only change the buffer before any I/O, or immediately after calling flush.

WARNING: The IO::Handle::setvbuf() is not available by default on Perls 5.8.0 and later because setvbuf() is rather specific to using the stdio library, while Perl prefers the new perlio subsystem instead.

WARNING: A variable used as a buffer by setbuf or setvbuf must not be modified in any way until the IO::Handle is closed or setbuf or setvbuf is called again, or memory corruption may result! Remember that the order of global destruction is undefined, so even if your buffer variable remains in scope until program termination, it may be undefined before the file IO::Handle is closed. Note that you need to import the constants _IOFBF, _IOLBF, and _IONBF explicitly. Like C, setbuf returns nothing. setvbuf returns ``0 but true'', on success, undef on failure.

Lastly, there is a special method for working under -T and setuid/gid scripts:


Marks the object as taint-clean, and as such data read from it will also be considered taint-clean. Note that this is a very trusting action to take, and appropriate consideration for the data source and potential vulnerability should be kept in mind. Returns 0 on success, -1 if setting the taint-clean flag failed. (eg invalid handle)


An IO::Handle object is a reference to a symbol/GLOB reference (see the Symbol package). Some modules that inherit from IO::Handle may want to keep object related variables in the hash table part of the GLOB. In an attempt to prevent modules trampling on each other I propose the that any such module should prefix its variables with its own name separated by _'s. For example the IO::Socket module keeps a timeout variable in 'io_socket_timeout'.


the perlfunc manpage, I/O Operators in the perlop manpage, the IO::File manpage


Due to backwards compatibility, all filehandles resemble objects of class IO::Handle, or actually classes derived from that class. They actually aren't. Which means you can't derive your own class from IO::Handle and inherit those methods.


Derived from by Graham Barr <>